The present invention relates to improvements to the performance and operation of anti-lock braking systems by employing non-contact position monitoring of an accumulator piston to control the volume of fluid retained within the secondary circuit of the braking system hydraulic unit.
Many known anti-lock devices operate by cyclically increasing and decreasing the braking force exerted on the wheels so that a wheel having a tendency to lock is permitted to re-accelerate back toward a speed corresponding to the speed of the vehicle. This is typically achieved by control valves alternately allowing fluid to flow out of and then into the brake cylinder to first lower and then raise the brake pressure in the brake system. Some anti-lock systems employ a pump-back scheme where fluid is dumped from the wheel cylinder to a local accumulator and the same hydraulic fluid is re-supplied from the local accumulator to the brake pad actuators. Simple ABS designs do not have the ability to generate wheel pressures in excess of master cylinder pressure, thus making them incapable of performing any active pressure events.
Most of such anti-lock braking systems are further capable of operating in a traction control mode. Traction control and anti-lock operation are both responses to aberrant vehicle wheel behavior. A traction control function is established by detecting conditions where the rotational speed of a first powered wheel substantially exceeds that of a second powered wheel. To provide a power balance in the operation of the vehicle, a braking force is applied to the powered wheel rotating at a higher speed to effectively transfer driving torque back to the other wheel that has better traction. Many anti-lock systems having such a traction control feature employ a motor and hydraulic pump or pumps which operate independent of the service braking system to supply fluid from a local accumulator to brake the wheel which has lost traction. The same local accumulator may be utilized during anti-lock operation and during a traction control event. In this case, it is highly desirable that the accumulator be sufficiently empty to receive braking fluid it is highly desirable that the accumulator be sufficiently empty to receive braking fluid dumped from a wheel cylinder during the anti-lock event and be sufficiently full to supply the needed fluid to the pump during a traction control event.
U.S. Pat. No. 5,971,502 is illustrative of such anti-lock braking systems with traction control (sometimes xe2x80x9cdynamic rear proportioningxe2x80x9d) capability. This patent also discloses a process for maintaining an optimum level of pressurized hydraulic fluid in a hydraulic accumulator. This optimum level allows the accumulator to immediately respond to a sensed aberrant wheel condition to receive fluid during an anti-lock braking event and to dispense fluid at the beginning of either a traction control event or back-up braking situations. The process includes a step of monitoring a plurality of vehicle operating parameters such as speed and recent braking activity (hydraulic fluid pressure) and utilizing those monitored parameters to periodically determining the likelihood of the occurrence of an anti-lock braking event. The amount of fluid in the accumulator is increased when the likelihood of an anti-lock braking event is low and is decreased when the likelihood of an anti-lock braking event is high. The likelihood of a traction control event may also be monitored and the amount of fluid in the accumulator increased when the likelihood of an anti-lock braking event is low and the likelihood of a traction control event is high.
The volume of fluid in an accumulator was also addressed by the patentee of U.S. Pat. No. 5,522,650, who, in an attempt to minimize motor xe2x80x9con-timexe2x80x9d and reduce vehicle noise and vibration, devised an algorithm for modeling the amount of fluid in the accumulator and enabled a motor to remove fluid from that local accumulator only as necessary. This patent is typical of all current designs in that software must make an estimation of the accumulator volume. This leads to increased motor running times, and reduced accuracy of pressure control algorithms.
Current designs require that the secondary circuit volumes be minimized in order to accurately estimate the suction path and maximize the through flow of this line in an ESP or TCS (traction control) event. Sophisticated algorithms are used to minimize the noise while maximizing the performance. Several ways currently exist to improve the performance of the ESP system, most including the addition of some volume in the suction path near the pump element inlet. Software algorithm development and implementation is limited to the information available through the electronic control unit (ECU) and its sensor array. Without an accurate measurement of the brake circuit volume, an estimation of this volume is utilized. If the secondary circuit volume could be measured, many improvements to the control algorithm could be implemented. The presence of an optimum amount of fluid in a secondary brake circuit residual reservoir can significantly improve pressure build times during traction control events.
There are many methods available for position monitoring. For example, International Application No. PCT/EP 01/12269 published as WO 02/36400 Ai teaches a Hall sensing element integrated into the master cylinder of a hydraulic brake system to sense the field of a magnetic region associated with the master cylinder piston for relatively rapid determinations of brake pedal position. Many other position sensing methods, such as linear variable differential transformers (LVDT""s) or various other mechanically actuated measuring techniques are also known, however they typically involve significant modifications to the existing hydraulic unit design box volume. Additionally, they are cost prohibitive, and do not provide a simple way to measure volume in the sealed designs already in production.
The present invention employs accumulator chamber management that is a unique and superior method to these designs. It provides a magnetic field sensor such as a Hall-effect solid-state sensing device or magneto resistive sensor to monitor the accumulator piston position to determine the amount of brake fluid contained in the accumulator chambers of an ABS hydraulic unit.
The accumulator chamber is a component of what is collectively referred to herein as the secondary braking circuit. The secondary circuit is that portion of the anti-lock braking modulator system that is employed during an anti-lock or traction event to make possible the necessary modulation of brake fluid pressure. During normal braking events such as moderate deceleration on a good road surface, the secondary circuit remains isolated from the primary hydraulic braking circuit that is used to transfer pressure from the master cylinder to the wheel cylinder.
The invention comprises, in one form thereof, a process of controlling the secondary hydraulic circuit of a vehicle braking system during aberrant wheel behavior, e.g., one wheel slipping excessively as in a traction control event or one wheel tending to lock during braking as in an anti-lock braking event. The process includes creating a magnetic field adjacent an accumulator in the secondary braking circuit and modifying the magnetic field in accordance with changes in the fluid volume within the accumulator. Changes in the magnetic field are monitored and those changes translated into an indication of accumulator fluid volume. The indication is then utilized in controlling operation of the secondary circuit, for example, by increasing the volume of fluid in the accumulator during anti-lock operation or supplying fluid to the pump from the accumulator to brake a slipping wheel as in a traction control event.
In another form, a process of braking a slipping one of a pair of differentially driven wheels to transfer driving torque therefrom to the other of the differentially driven pair of wheels includes maintaining a preferred volume of brake fluid in an accumulator, monitoring the rotational speed of each of the driven wheels, identifying one wheel speed as deviant if the monitored speed thereof becomes too much larger than that of the other driven wheel, and closing a normally open valve which provides a path for the application of normal braking fluid pressure to a brake actuating mechanism associated with the non-deviant wheel to isolate the non-deviant wheel from fluid braking pressure. A pump is then energized to supply brake fluid from the accumulator to a brake actuating mechanism associated with the deviant wheel to slow the deviant wheel, which allows the transfer of driving torque to the non-deviant wheel. Monitoring of the deviant wheel speed continues and the pump is periodically energized and de-energized to reduce the supply of brake fluid to the brake actuating mechanism associated with the deviant wheel. The pump is disabled and the previously closed valve is allowed to open upon an indication that the deviant wheel has achieved a target speed. To accommodate extreme cases, the volume of fluid remaining in the accumulator continues to be monitored and a further valve is opened to supply brake fluid from a master cylinder to the pump inlet when the monitored accumulator volume reaches a minimum value.
An advantage of the present invention is that the ability to calculate accumulator volume facilitates both the software development and production control algorithms by providing an accurate accounting of the amount of brake fluid in the secondary circuit of the braking system.
Accumulator chamber management allows for the use of proven components that are significantly less expensive than the current ESP designs. Thus, another advantage of the present invention is that modifications to the hardware of existing hydraulic braking systems to incorporate the present invention are minimal.
A further advantage is through the use of secondary circuit brake fluid management with pulse width modulation motor drive and accumulator position monitoring, significant performance enhancements can be achieved using less complex hydraulic components.
An advantage of one form of the present invention is based on the use of controlled accumulator piston stroke to manage the secondary circuit volume for ABS (anti-lock) hydraulic units thus allowing for PBC (panic braking) and partially active ESP (electronic stability programs) to be performed without the need of isolation and suction valves.
As another advantage, the volume of brake fluid in the accumulator provides the additional benefit of acting as a spring loaded pre-charge for the pump element, optimizing the pump performance throughout the temperature envelope and eliminating the need for a supplemental pressurized brake fluid accumulator. This allows the use of an inexpensive pump for all designs.
A further advantage of the present invention is that the cost of providing direct accumulator volume information is quite modest.